In the art of mass data storage, it is to be expected that occasionally stored data becomes unreadable on the medium on which it is stored. A common reason for the data becoming unreadable is that the write mechanism loses correspondence with the read mechanism. When this happens, although the data as stored may have full integrity, the read mechanism is simply unable to retrieve the data as stored, since the write mechanism has deposited the data on the medium in a different location from the location where the read mechanism expects to find it.
Standard algorithmic data recovery techniques such as Error Correction Code ("ECC") are typically unable to recover data lost in this way. Since the read and write mechanisms are out of correspondence, there is generally too much data missing for ECC to recover mathematically. In such cases, heroic data recovery techniques must be resorted to. Off-track positioning is the primary heroic technique used to recover data suspected to be lost through loss of correspondence between read and write mechanisms. If off-track positioning fails, the success of other heroic techniques such as modifying read channel characteristics (read bias, filter boost, etc.) or re-tensioning the tape (in the case of tape data storage) becomes very unpredictable.
Standard off-track positioning techniques typically employ a predetermined sequence of read track adjustment steps where, starting at the track centerline (where the data is expected), the read head is displaced in successively larger off-track distances symmetrically either side of the centerline until the data is discovered. This methodology assumes that the small misalignments between read and write mechanisms are more likely to occur than large misalignments in every case.
This "centerline increment" approach is flawed for at least two reasons. First, the forces that actually work to produce read/write mechanism misalignment include contributions from conditions where a large misalignment may be just as likely as a small misalignment. For example, a primary cause of misalignment is a "wander" between write head and storage medium. This "wander" is just as likely to be large as small, depending on the effects that are causing it. The "wander" may, however, be measured and subsequently predicted with some accuracy according to particular characteristics of the data storage device.
Second, the "centerline increment" approach tends to magnify the effect of hysteresis inherent in the reciprocal movement of the read head. Most read head mechanisms employ meshing worm gears to enable small but repeatably accurate displacements across the storage medium. Slight imperfections (or normal wear and tear) in the gear profiles cause "gear lash", where, over time, "slop" in the gears diminishes the actual displacement of the read head for a given rotation of the drive worm gear. This hysteresis effect is generally optimized by biasing the meshing worm gears, normally with a spring. Nonetheless, this hysteresis effect always tends to have the most impact on small movements of the read head.
The hysteresis effect is thus magnified in the "centerline increment" approach to off-track positioning, since the approach starts with small displacements either side of the centerline, and progresses to larger increments (where hysteresis may have a less pronounced effect) only when small displacements have failed to retrieve the data. Indeed, under the "centerline increment" approach in a device where hysteresis is pronounced, the off-track positioning mechanism may not actually move the read head off the track centerline for several consecutive incremental off-track read attempts. In such cases, the time to recover data via off-track positioning is clearly extended.
There is therefore a need in the art to conduct data recovery via off-track positioning in accordance with predictable misalignment characteristics of the storage device rather than by some arbitrary approach such as "centerline increments". It would be further advantageous if improved off-track positioning techniques minimized the effect of hysteresis in the read head mechanism.